Studies on the structure of macromolecules in cancer cells are essential for defining the biochemical lesion(s) distinguishing cancer cells from normal cells. The present research proposal concerns 3 related topics: (1) A methodological investigation on postlabeling procedures for the sequence analysis of nonradioactive RNA; (2) Sequence analysis of specific 5-methylcytidine-rich tRNAs from normal and cancerous rat and human tissues; (3) Studies on the effects of an anticancer drug, 5-azacytidine, on RNA and DNA. Comments: (1) Structural analysis of small amounts of nonradioactive nucleic acids requires incorporation of radioactive label by "postlabeling" techniques. Methods to be developed will entail a combination of 3H- and 32P-labeling, digestion with nucleases, and thin-layer separations. Each position of the polynucleotide chain will be identified directly as a radioactive derivative. (2) We wish to determine the structure of tRNAs that are rich in 5-methylcytidine because 5-azacytidine was found in our laboratoy to inhibit specifically the biosynthesis of this methylated compound in mammalian tRNA. (3) Preparation of nucleic acids deficient in a single modified constituent such as 5-methylcytosine will enable us to define the role(s) of this constituent. In vivo studies are planned to compare the effects of 5-azacytidine on tRNA in various normal and neoplastic tissues and on DNA in L1210 leukemic cells, while in vitro studies will be aimed at identifying the biological function(s) of 5-methylcytidine in tRNA of normal and neoplastic cells.